Revealed: What happens when rivers get too hot for fish
Rising river temperatures linked to climate change prompted Inland Fisheries Ireland (IFI) to introduce a warm water protocol to protect cold water species like Atlantic salmon, sea trout and brown trout in the River Moy, which flows through Co Mayo.
“The protocol has been in place for the past two years,” said Seán Kelly, a research officer with IFI who was involved in developing the scientific advice for the warm water protocol.
“Luckily, in 2024, we had a relatively mild summer. It was quite wet and cool so did not necessitate any fishery closures.”
Last year, Mr Kelly said, was very different: “We had a couple of periods of very warm and dry weather. According to the scientific advice formulated as part of the protocol, it necessitated fisheries to close.”
The Errif and Moy fisheries closed on three occasions last year – in late May, late June and late July. Each closure lasted less than a week.
“Once water temperatures begin to encroach on 20 degrees, there is a need to try and minimise the amount of handling and additional stress that cold water species experience,” Mr Kelly said.
People fishing in the River Moy in Ballina, Co Mayo. Photo: Getty
“Once temperatures go above 18C, we are in that risk zone where a closure may be imminent. Once temperatures exceed 20 degrees, which is known to increase sub-lethal or even lethal effects related to angling on these species, the fishery can close.
“Water temperatures are checked at intervals throughout the day and, if a reading shows that the temperature is over 20C for two readings within a 24-hour period, then that closes the fishery.”
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Mr Kelly explained that salmonoid fish species are cold-water adapted. “We should think of them as polar species,” he said.
“Here in Ireland, we are very close to the southern limit of the distribution of those species.”
As summers warm, therefore, the narrow window of suitable habitat could shrink even further.
In relation to the wider conservation challenge, Mr Kelly said: “The trend is bad, particularly in recent decades. Numbers do vary river to river, but during the 1970s there were over 1.5 million salmon returned to Ireland, as an estimate, and that’s down to less than 200,000 in recent years.”
Mr Kelly explained the physiological effects that take place when cold water fish are exposed to temperatures above 20 degrees.
Most anglers want to preserve fish stocks. Photo: Getty
“Unlike mammals, they can’t regulate their internal body temperature,” he said. “It changes as the ambient water temperatures change. So once they enter into this state of stress, they must adapt their behaviour. This is referred to as thermoregulation.”
During thermoregulation, the fish’s movements reduce. They also eventually stop feeding.
“That is their coping strategy and that is why it is so critical that we minimise any additional stress that can exacerbate that compromised state they’re in,” Mr Kelly said.
In terms of the effectiveness of the protocol, Mr Kelly feels positive.
“Having the fisheries closed during periods when river temperatures were in excess of 20C, we can say with a high degree of certainty we would’ve minimised any additional unnecessary injuries or mortality that would’ve occurred,” he said.
“In terms of the closures themselves, I don't think they will reduce the demand for angling. Most anglers are ultimately conservationists. They’re passionate about ensuring the long-term sustainability of the stocks continues into the future.”